The Beatles were the most influential rock band of the 1960s, releasing a string of albums that had a profound impact on modern music. As the group progressed from pop ballads to psychedelic rock, they sold tens of millions of records.

The group has long been a digital holdout -- choosing for years to sell their music exclusively on CDs, cassette tapes and records, even as fans moved to digital platforms.

The Beatles' music finally arrived for purchase on Apple's iTunes service in 2010, selling 2 million songs during the debut week, according to Billboard.

Former Apple(AAPL) CEO Steve Jobs was said to be a Beatles fan, and negotiated for years with EMI Group, the Beatles' record label, and Apple Corps, the Beatles' business group.

The company long claimed that the reason its music wasn't on iTunes was that digital music was of inferior quality to CDs and records. But legal issues may have also been a factor.

In 1978, Apple Corps sued Apple Computer for trademark infringement. As part of a settlement reached three years later, Apple Computer agreed not to enter the music business.